The invention relates to tamperproof closures, and in particular the type having an annular heat-shrinkable band depending from the lower edge of the skirt. Such tamperproof closures typically have a circular panel, and a threaded annular skirt depending from the panel. After the closure is applied to a bottle neck, heat is applied to the tamperproof band depending from the skirt causing the tamperproof band to shrink into shape-conforming engagement with the bottle neck. When the closure is removed from the neck, the tamperproof band breaks away on a circumferential weakened tear line at its attachment to the skirt. The condition of the tamperproof band can therefore be used to indicate whether there has been an attempt to tamper with or open the container.
The molding of such closures presents some difficulties, however. The desired weakness at the attachment of the tamperproof band to the skirt, which permits the tamperproof band to be torn from the skirt, also renders the closure relatively fragile during the molding operation. It cannot withstand any severe stripping action during removal from the mold. Furthermore, the tear line is typically formed by an annular groove or a circumferential line of perforations. Mold members which create the groove or perforations interfere with the axial removal of the closure from the mold. Such interference could, of course, be eliminated by the use of a radially opening mold. However, as such radially opening molds are relatively complicated and expensive, it is generally desirable to avoid their use where possible.
A closure design which eliminates the necessity of a radially opening mold is disclosed in U.S. Pat. No. 4,033,472 to Aichinger, assigned to Albert Obrist AG. The so called Obrist closure disclosed in the Anchinger patent has a tamperproof band which tapers from a thin sectioned connection at the lower edge of the skirt down to a thicker lower portion of the tamperproof band. The Obrist tamperproof band is integrally attached to the inside circular lower edge of the skirt.
The tamperproof band of the Obrist closure has an outside diameter which increases as the axial distance from the lower edge of the skirt increases. Therefore, the tamperproof band interefers with the axial removal of the mold member which defines the outer surface of the tamperproof band. Due to its thinness and flexibility, however, the tamperproof band is deformable to permit the axial removal of the mold member. A smoothly contoured taper of the tamperproof band facilitates this deformation and the stripping of the closure from the mold. Even so, such stripping action is an undesirable, relatively unreliable and troublesome step in the manufacturing process. Moreover, if perforations are to be molded into the closure to form the tear line, the problem of interference between the mold and the tamperproof band increases.
When the weakened line of attachment between the tamperproof band and the skirt is perforated, the thickness of the tamperproof band becomes more critical. During the molding process, the flow of thermoplastic material into the mold cavity defining the tamperproof band is only through the relatively narrow bridges between the perforations. Relatively narrow streams of thermoplastic material flow through these bridges and expand into the cavity beyond to form a tamperproof band. The lines where these expanding streams again meet are referred to as "cold-weld lines", and form an undesirable weak point in the tamperproof band. If the tamperproof band breaks prematurely on weakened cold weld lines, the band is no longer effective to indicate whether there has been an attempt to remove the closure. The thickness and taper of the tamperproof band is critical both for minimizing the cold weld problem, and for providing the required heat-shrinkable properties. Yet the taper required for permitting axial separation of mold members forming an Obrist type closure may not be ideal also for minimizing the cold weld problem.
A useful improvement in an Obrist type closure reduces or eliminates the stripping action of interfering mold members and closure portions, provides for the possibility of a tamperproof band having a uniform thickness, or other desired taper, and allows the closure to be designed for different degrees of tearability, without requiring an expensive mold structure having radially opening components.